I am totally blocked at making a chain Behat 3 / Firefox 55 (64 bits) / Gecko driver (0.18) works. The browser is launched, the page (base_url) is loaded, but no matter what the content of the HTML ($session->getPage()->getHTML()) is null

any clue about what can be going wrong? (I can share the whole projet easily, it is a sample on google home page)

Selenium2 was changing the protocol being used, which is why it needed a different driver than Selenium 1. Selenium 3 is an evolution of version 2, so our driver covers it too (too bad for the name)

For GeckoDriver with Selenium 3 though, you may get better result using the dev version of Selenium2Driver, as we have unreleased improvements for it (GeckoDriver implements the W3C WebDriver protocol, not the Selenium WebDriver protocol, so there are a few differences)